[Android][SysApps] Change event names according recent spec changes.

    Refer to https://github.com/sysapps/device-capabilities/issues/14, the event
    names changed as below:
        attribute EventHandler  onstorageattach;     // was onattach
        attribute EventHandler  onstoragedetach;     // was ondetach
        attribute EventHandler  ondisplayconnect;    // was onconnect
        attribute EventHandler  ondisplaydisconnect; // was ondisconnect
    
    At the same time, fixed a bug to allow pass null as callback of "onEventName".